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the Word on back pressure?
I've been noticing a lot of posting concerning exhaust/headers recently. After some searching, most of the info I can find concerning back pressure at the exhaust manifold/head suggests that freeing it up is a great thing.
A buddy of mine said that's only the case at higher rpms, and at lower rpms backpressure helps with torque. Searching I've seen concerning exhaust set-up seems to concur: wider diameter piping plus cat-free setups can sacrifice low-end torque, etc. I'm hoping to get some definitive science/garage-pseudo-science on this backpressure stuff. Any bites?? |
backpressure is bad PERIOD. It doesn't help with anything, ever! Anyone that tells you backpressure helps with anything is WRONG.
Where you get help with lower end torque is by not running the maximum recommended exhaust size ;) i.e. a 2.4L 4cyl should run 2" or 2.25" for maximum velocity but reduction of backpressure without TOO much flow to eliminate low end torque. 2.5" would work better for top end (to a point of course) |
The way I understand it, both a more restrictive exhaust and intake flow causes the air to move faster. This means that at low rpms, the spent charge is more fully exhausted and the intake fills the chamber more completely with air. If you can picture in your mind a slow-moving piston with varying orifice sizes, you can see how a smaller opening causes the air to be pulled harder into the chamber.
Obviously at higher rpms restrictive flows are bad for power. |
Originally Posted by chimmike
(Post 50484078)
backpressure is bad PERIOD. It doesn't help with anything, ever! Anyone that tells you backpressure helps with anything is WRONG.
Where you get help with lower end torque is by not running the maximum recommended exhaust size ;) i.e. a 2.4L 4cyl should run 2" or 2.25" for maximum velocity but reduction of backpressure without TOO much flow to eliminate low end torque. 2.5" would work better for top end (to a point of course) "Where you get help with lower end torque is by not running the maximum recommended exhaust size " um, yeah, because you need some backpressure. why do you think you lose torque with too big of exhaust? you contradict yourself from one line to another |
Originally Posted by mochester
(Post 50484186)
"Where you get help with lower end torque is by not running the maximum recommended exhaust size "
um, yeah, because you need some backpressure. why do you think you lose torque with too big of exhaust? you contradict yourself from one line to another |
Here's Gadget's explaination from TTORA:
Every time I hear this crap that our engines need back pressure I just shake my head. It is just the dumbest thing around. The Otto cycle engine does not need any backpressure. In fact the less backpressure the better. A suction in the exhaust is even better as it will help pull exhaust out of the cylinder so the engine will not have to use power to push it out. Check this out, airplanes fly. They do this by managing flow dynamics also known as fluid dynamics. When you increase the velocity of a fluid its pressure decreases. When this decrease in pressure is above the wing, the pressure under the wing pushes up lifting the plane into the air. When you decrease the velocity its pressure increases. When air is drawn through a venturi in a carb the pressure drops and that is what draws the gas out of the bowl and into the air stream. If you look at a venturi you would think that it is a big restriction, but it causes the pressure to drop. A venturi is basically an airplane wing in a circle. Now with an exhaust system you want the lowest pressure you can get so the engine works less hard pushing the exhaust out. So, it is all about flow management. A small pipe will cause the flow to be faster and it travels down that pipe in pulses. The faster it goes the less the pressure is especially after the pulse wave. If you get to big of a pipe the velocity slows down and the pressure increases. So a large diameter pipe can in fact result in a higher pressure in the pipe and cause the engine to work harder to push out the exhaust and you loose torque. Do you want a visual aid? If so, light two candles. Place them about 6? apart. Now blow between them. You will notice that the flames lean toward the air that you are blowing between the candles. This is because the pressure between the candles dropped and the higher-pressure air is rushing to move toward the low-pressure air pushing the flames over as it passes through. Now there are always compromises. Your engine runs at different RPMs and different loads and you will want the smallest diameter pipe possible without restricting high flow at high RPM and loads. At some point a small diameter pipe will become flow restrictive and will need to be larger, but the trade off is loss of low load velocity and increased pressure and you perceive this as loss of low RPM torque. A short straight pipe is best but you have packaging problems. If you have to have bends, the bends should be a larger diameter then the straight sections because you want the flow to slow down to reduce turbulence in the bend, but this really adds huge cost to the system. Now if you really want something slick, have all your bends take on the NACA duct peanut shape for the best possible flow. Mandrel bending prevents a smaller diameter then the straight and is usually the best you get. It is all about managing flow velocities without becoming flow restrictive. With headers designs, if you can decide what RPM you want the most power, you can tune your primary tube length so that the lower pressure portion of the pulse wave of one cylinder is passing the end of the primary tube that has the exhaust valve that is just opening so that primary tube will have the lowest pressure possible. This however can only be tuned to a specific RPM range and you also have to worry about packaging problems. Usually headers with big fat primary tubes will perform worse then ones with smaller tubes. Now when you hear someone say that you need back pressure for low RPM torque they are exactly opposite of what is really going on. You want the least pressure possible in the pipe at low RPM (or any RPM) and larger pipe increases pressure because the velocity is much lower. None of this applies to turbo systems. That is a whole other chapter on exhaust theory. __________________ Gadget |
Originally Posted by engnbldr
But now we need to think about the exhaust system. Improving the exhaust system is number one as far as freeing up usuable power, it doesn't "make" any at all. But the factory needed a quiet, smooth, and responsive engine because the driver might be a little old lady going to the store in Hawaii, or a young fellow driving around at 5000 ft altitude in Colorado. So they compensated for this, and for ground clearence on some vehicles.
The best setup cost wise we have found is to use the excellent factory exhaust manifold, then increase the pipe size to 2" all the way to the muffler, yep, the cat, too. Then we increase again to 2 1/4" on exit all the way back. This creates a bit of directional flow and frees up power, it fairly quiet and smooth. http://www.4x4wire.com/forums/showfl...5&o=14&fpart=1 I was interpreting this to mean that leaving the stock manifold leaves some good back-pressure, but gently opening the exhaust wider creates a directional flow compensating for the initial back pressure. Maybe I'm wrong in my thinking here, and the key word is "best setup cost wise." And the lack of header in that write-up is due to cost. Thoughts? |
Well I hate to speak for EB, he's forgotten more than I'll ever know. But maybe cost and the extra problems that come with headers.
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back pressure is bad. you dont want it. however, when someone says you need "backpressure", what they usually mean is you need exhaust gas velocity, which in turn helps with exhaust scavenging (sucking). the smaller the diameter of the pipe, the faster the gasses flow through. the size of pipe you use depends primarily on engine size and whether or not your turbo.
in my experience, as has been stated in this thread already, the best way to free up power from exhaust is by minimizing the amount of bends and restrictions to flow, in other words, the straighter the exhaust, the better, the less mufflers and cats, the better. increasing ehxuast diameter is not the best idea unless you are turbo |
thank you. Again, anyone who says you need backpressure is wrong.
Velocity is good. The faster the gas can exit wtihout lingering, the better. Obviously if you use TOO small an exhaust it will be restrictive and bad. You must find the optimum size for n/a. If you run a cat and resonator and muffler that's a good time to run slightly larger piping due to the restrictions you're adding. For turbo, Bigger is always better, period. |
I'll say it, and no it's not wrong: You may need some back pressure. You risk burning the exhaust valves and actually decreasing the mileage without a little back pressure. What you don't want is excessive back pressure.
The reason you *may* need some back pressure (and this really depends on the overlap in your cam) is that during the combustion cycle, both the intack and exhaust valves are open at the same time. Without a little back pressure, raw fuel can pass directly thru the cylinder and into the exhaust ports. This lowers the fuel economy and can cause damage to the exhaust valves because there is now a flame in the exhaust port from the passed fuel. All of this depends on the valve timing, the amount of valve overlap and the size of the exhaust headers. It's not a simple "one size fits all" answer. |
Originally Posted by InternetRoadkill
(Post 50484415)
I'll say it, and no it's not wrong: You may need some back pressure. You risk burning the exhaust valves and actually decreasing the mileage without a little back pressure. What you don't want is excessive back pressure.
The reason you *may* need some back pressure (and this really depends on the overlap in your cam) is that during the combustion cycle, both the intack and exhaust valves are open at the same time. Without a little back pressure, raw fuel can pass directly thru the cylinder and into the exhaust ports. This lowers the fuel economy and can cause damage to the exhaust valves because there is now a flame in the exhaust port from the passed fuel. All of this depends on the valve timing, the amount of valve overlap and the size of the exhaust headers. It's not a simple "one size fits all" answer. |
No, I'm saying that depending on your valve timing and other considerations, you may need some back pressure to prevent the cylinder from being over scavenged which can cause the incoming fuel-air mixture to be pulled into the exhaust ports.
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roadkill, I think in your instance the only time that applies is when running an open header. If you're running a full exhaust, that does not become an issue as there's not enough scavenging to cause the issue.
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Originally Posted by InternetRoadkill
(Post 50484458)
No, I'm saying that depending on your valve timing and other considerations, you may need some back pressure to prevent the cylinder from being over scavenged which can cause the incoming fuel-air mixture to be pulled into the exhaust ports.
http://www.break.com/index/ford_recalls_trucks.html |
Originally Posted by chimmike
(Post 50484534)
roadkill, I think in your instance the only time that applies is when running an open header. If you're running a full exhaust, that does not become an issue as there's not enough scavenging to cause the issue.
Flowmaster has a nice write-up on the effects of back pressure: http://www.flowmastermufflers.com/backpressure.html The article mentions that converting to a high-flow exhaust may require changing the cam to reduce the amount of valve overlap for best results. |
Originally Posted by chimmike
(Post 50484534)
roadkill, I think in your instance the only time that applies is when running an open header. If you're running a full exhaust, that does not become an issue as there's not enough scavenging to cause the issue.
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Originally Posted by mt_goat
(Post 50484200)
Here's Gadget's explaination from TTORA:
the exhaust gasses are only moving slower in a bigger pipe, anyways, BECAUSE there is less pressure forcing them to exit the pipe to make room for more exhaust. in fact, yeah dude, i don't buy it at all there is no way bigger exhaust equals more pressure. just because the gas moves slower doesn't mean there is more pressure. the speed of the exhaust gasses moving through the pipe is dependent on the pressure forcing it through the pipe, not the other way around. this guy has it totally backwards and you bought it think about it. get different diameters of pipe and blow through them. from really skinny, like the size of one of those tiny coffee straws, to really big, like an inch in diameter or bigger. tell me which has more backpressure. he is right about the gasses moving slower in a bigger pipe. but he is totally wrong in assuming that it means there is more pressure back pressure is good and without it low end torque is lost, period |
Originally Posted by notanymore
(Post 50484702)
so are you saying that running no piping after the headers can cause problems? because right now im just running straight off my headers with no piping connected to them (because the shops here arnt the best and have been unable to get it piped up, not because i want it that way)
As for a high flow exhaust causing the same thing on motors, i've never seen it happen, ever. There just isn't enough overlap on stock cams or even aftermarket cams to cause this issue on a vehicle with a full exhaust setup. |
Originally Posted by mochester
(Post 50484734)
back pressure is good and
No, backpressure is not good, EVER. Backpressure does not make torque. Backpressure hurts power AND torque. Optimum velocity produces torque AND horsepower. In order to achieve optimum velocity, you must eliminate backpressure as best you can. http://en.wikipedia.org/wiki/Back_pressure http://auto.howstuffworks.com/question395.htm http://www.flowmastermufflers.com/backpressure.html http://www.enjoythedrive.com/content/?id=8179 NONE of those resources say backpressure makes torque, and NONE of them say backpressure is good. So, educate yourself. Backpressure does NOT make torque. |
Originally Posted by mochester
(Post 50484734)
ok, i have a couple problems with this guy's argument that bigger exhaust = more pressure. first, his argument about the candles makes no sense, and does not relate to exhaust gasses moving through a pipe at all. second, i want to see some sort of test that proves that bigger exhaust pipe = more pressure. sure, you can rationalize it in your mind by saying that the air moves slower through a bigger pipe. but i need numbers. i need some sort of barometer device inside different diameters of exhaust pipes. because that explanation doesn't work for me.
the exhaust gasses are only moving slower in a bigger pipe, anyways, BECAUSE there is less pressure forcing them to exit the pipe to make room for more exhaust. in fact, yeah dude, i don't buy it at all there is no way bigger exhaust equals more pressure. just because the gas moves slower doesn't mean there is more pressure. the speed of the exhaust gasses moving through the pipe is dependent on the pressure forcing it through the pipe, not the other way around. this guy has it totally backwards and you bought it think about it. get different diameters of pipe and blow through them. from really skinny, like the size of one of those tiny coffee straws, to really big, like an inch in diameter or bigger. tell me which has more backpressure. he is right about the gasses moving slower in a bigger pipe. but he is totally wrong in assuming that it means there is more pressure back pressure is good and without it low end torque is lost, period I like the way you're standing your ground and saying prove it to me, and there my not be anything I could say or do to convince you as I'm far from totally understanding all the dynamics of what's going on myself, (this kind of reminds me of the question "which came first the chicken or the egg?") lol.:hillbill: With any big debate there is one main point that is confusing or misunderstood. Like back when people thought the earth was flat and some said no, it's round. In that case, the main point of misunderstanding was the gravitational force of a planet, lol. Here IMHO the main point that's hardest to visualize is the fact that air has mass, and as such is subject to Newton's laws of motion. What often helps me grab a concept is to take the subject to the limit, so here goes an attempt at that. Think of the exhaust gases in the pipe as a coal train, ok, now you can picture some big mass. And the train has 3 locomotives or it could have 4 too if you want, or 6 or 8, but if it has 6 or 8 you have to split it into two trains at first and merge them together down the tracks into one train which is harder. And the locomotives instead of giving constant power are only allowed to give pulses of power, one at a time, with a brief pause in between each pulse where there is no power to move the train and the only thing keeping it moving is inertia. Now we have an exhaust train, and like the coal train it can be light (like with no coal loaded, just empty cars) or it can be heavy (like loaded down with coal). Now our exhaust train it is heaver when it's cold and light when it's hot, (or that's my theory anyway) So when we step up to a bigger pipe we not only have more mass to push down the tracks (ie bigger railcars now) but we slow the stream down and allow it to cool more and get heavier. I'm pretty tired now, think I'll go home. More later maybe.:bigok: |
This is complicated, so I'm going to grossly oversimplify. A larger pipe can cause higher back pressure... sort of. The reason is that a smaller pipe produces higher velocities and hence higher momentum of the outflowing exhaust gas. When the exhaust valve closes, the momentum prevents the outflowing gas from stopping suddenly. The result is that a momentary vacuum forms near the exhaust port. Headers are designed to promote this effect.
The trick in sizing exhaust is to choose a size small enough to promote a high velocity flow, but not so small that it begins to obstruct the flow. |
Personally, I ascribe to the "Exhaust Gnome Theory."
Think about it...the bigger the pipe, the more exhaust gnomes you can fit into the area. As with any other environment, too many hands generally become ineffeicient,too many gnomes = lack of personal touch = loss of efficacy thus proving that bigger pipes equal loss in effectiveness due to the ability of the gnomes to slack off... Smaller pipe = more industrious gnome who knows he is gonna get his ass replaced if he does't work hard enough. he is efficient because he has to be. You see, all the physics and holistic mumbo jumbo aside, it all really boils down to motivation. one highly motivated gnome is much more effective than many un-motivated lazy gnomes. I believe this clears up the entire problem...:guitar: On a serious note, the only time I have ever heard of more flow damaging an engine is running open headers for any period of time. I am afraid i don't have a 10lb brain so I can't give you specifics, but i figue that the guys who designed the exhaust system had something going for them. If you are looking to spend eight million dollars on an exhaust for a 22r...do what you gotta do. But as far as general everyday use goes i think this argument is a little bit far out there. I have really liked this thread as it seems everyone has put a lot of time and research into their posts, except me of course. Thanks for all the good info, whatever side you are on!!!!! Jon |
Originally Posted by InternetRoadkill
(Post 50484965)
A larger pipe can cause higher back pressure... sort of. The reason is that a smaller pipe produces higher velocities and hence higher momentum of the outflowing exhaust gas.
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Originally Posted by DaveInDenver
(Post 50485040)
It's called Bernoulli's Principle. Velocity and static pressure are inversely related, so as velocity goes up, static pressure goes down.
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Originally Posted by InternetRoadkill
(Post 50485054)
Actually, Bernoulli's Principle is not what is at work here. Bernoulli's applies to venturis.
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html Another graphic of Bernoulli Effect, plus you can see how velocity, pressure, area relate. http://home.earthlink.net/~mmc1919/venturi.html |
Originally Posted by DaveInDenver
(Post 50485088)
A venturi is one demonstration of the Bernoulli Effect, but so is an aircraft wing, the nozzle on a rocket engine, a curve ball. Bernoulli simply described the phenomenon that as fluid velocity increases, the pressure goes down.
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html Another graphic of Bernoulli Effect, plus you can see how velocity, pressure, area relate. http://home.earthlink.net/~mmc1919/venturi.html |
Actually, I'm familiar with the Bernoulli effect. But as I mentioned, it applies to venturis, not constant-area exhaust pipes.
The Bernoulli effect does apply to aircraft wings (think of them as half of a venturi), but it doesn't apply to rocket nozzles (Bernoulli only works with incompressible flows). A curve ball is an indirect result of Bernoullis. The curve is produced by the ball's rotation influencing the boundary layer of the flow across its surface. Incidently, pressure doesn't drop just because the air is moving. You can have a high-velocity flow with the same (or even higher) pressure as a slow moving flow. Bernoulli's describes the change in pressure/velocity as the ratio of potential/kinetic energy changes. But this requires that the total energy be constant. This is not the case for an exhaust flow which is receiving the residual combustion blast and waste heat from the cylinder. |
i'm going to go with the one that i can test for myself on a scale model. it should be very clear that when blowing through first a drinking straw and then a section of garden hose, which one has more backpressure
edit: that part about blowing between the two candles still makes me laugh its totally irrelevant. he doesn't even attempt to explain how it relates to an exhaust pipe. that's the main reason i don't buy anything this guy says. he can't even give an example, and his explanations don't make sense. maybe i'm just not smart enough, or maybe he's full of ☺☺☺☺. air doesn't act the same way trains on tracks do. that's a horrible example too. air doesn't move on tracks, connected to other air in strings. air gets compressed and expands and bounces all over the place. not one example that works as well as mine, which disproves the bigger exhaust=more pressure theory in 2 seconds. what's a better measurement of backpressure than you're own lungs? think about it. they're like a big air pump, and you can be the computer, analyze what you feel, and make a decision about what has more backpressure - a smaller or larger diameter tube. if you have any common sense, you won't even need to test this, and you will conclude what i have concluded unless you can think of an example for your theory that actually involves air moving through a pipe |
Originally Posted by InternetRoadkill
(Post 50485138)
The Bernoulli effect does apply to aircraft wings (think of them as half of a venturi), but it doesn't apply to rocket nozzles (Bernoulli only works with incompressible flows).
http://www.fluidmech.net/tutorials/b...-bernoulli.htm Look, I'm not going claim I know much about the thermodynamics of an auto exhaust (or heck, thermo in general, I only took two classes in it anyway), I was just pointing out the physical reason. The sizing of an auto exhaust is something I'll trust the experience that people have found before. I'm more of a Maxwell, Frick, Moore and Faraday kinda person anyway. But I do happen to agree that there is an optimal balance of velocity, pressure and volume that tunes the exhaust to the engine. Two stroke guys have known that for a really long time. |
Wedge a potato in your exhaust and see if thats too much back pressure :hillbill: .
I am totally lost on this post haha |
Turbines work better than potatoes.
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word. some deep stuff here. ive been sonfused on this issue as well. this kinda helped. i think the only real solution is to turbo everything. lol.
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Originally Posted by mochester
(Post 50485157)
i'm going to go with the one that i can test for myself on a scale model. it should be very clear that when blowing through first a drinking straw and then a section of garden hose, which one has more backpressure
edit: that part about blowing between the two candles still makes me laugh its totally irrelevant. he doesn't even attempt to explain how it relates to an exhaust pipe. that's the main reason i don't buy anything this guy says. he can't even give an example, and his explanations don't make sense. maybe i'm just not smart enough, or maybe he's full of ☺☺☺☺. air doesn't act the same way trains on tracks do. that's a horrible example too. air doesn't move on tracks, connected to other air in strings. air gets compressed and expands and bounces all over the place. not one example that works as well as mine, which disproves the bigger exhaust=more pressure theory in 2 seconds. what's a better measurement of backpressure than you're own lungs? think about it. they're like a big air pump, and you can be the computer, analyze what you feel, and make a decision about what has more backpressure - a smaller or larger diameter tube. if you have any common sense, you won't even need to test this, and you will conclude what i have concluded unless you can think of an example for your theory that actually involves air moving through a pipe Dude, you are thick. It states here in this thread about 5 times that the ideal pipe size would be the small as you can go without restricting flow. Obviously the drinking straw is too small for your lungs, or else it would be a PITA to drink out of. Your lungs are not a good comparison, because your lungs will NEVER put out enough air to displace all of the air in anything such as a garden hose. The candle example is a PERFECT example of how air traveling faster loses density. Which relates to how easily it can be expelled from the exhaust. You want to keep the air moving fast, becuase when it slows down it thickens. Too big of an exhaust also allows the air to cool, which is also makes it thicker. I could explain much more about this, but it would be repetetive considering it has all already been said here once or twice in this thread. |
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